1. Field of the Invention
This invention relates to an apparatus containing specific binary combinations of glycosyltransferases, for the synthesis of specific saccharide compositions such as, for example, oligosaccharides, polysaccharides and glycopeptides.
2. Discussion of the Background
The term "carbohydrate" embraces a wide variety of chemical compounds having the general formula (CH.sub.2 O).sub.n, such as polysaccharides. Oligosaccharides are chains composed of saccharide units, which are alternatively known as sugars. These saccharide units can be arranged in any order and the linkage between two saccharide units can occur in any of approximately ten different ways. As a result, the number of different possible stereoisomeric oligosaccharide chains is enormous.
Of all the biological polymer families, oligosaccharides and polysaccharides have been the least well studied, due in considerable part to the difficulty of sequencing and synthesizing their often complex sugar chains. Although the syntheses of oligonucleotides and polypeptides are well developed, corresponding synthetic techniques for synthesizing oligosaccharides have been slow to develop.
Numerous classical techniques for the synthesis of carbohydrates have been developed, but these techniques suffer the difficulty of requiting selective protection and deprotection. Organic synthesis of oligosaccharides is further hampered by the lability of many glycosidic bonds, difficulties in achieving regioselective sugar coupling, and generally low synthetic yields. These difficulties, together with the difficulties of isolating and purifying carbohydrates and of analyzing their structures, has made this area of chemistry a most demanding one.
Much research effort has been devoted to carbohydrates and molecules comprising carbohydrate fragments, such as glycolipids and glycopeptides. Research interest in such moieties has been large due to the recognition that interactions between proteins and carbohydrates are involved in a wide array of biological recognition events, including fertilization, molecular targeting, intercellular recognition, and viral, bacterial, and fungal pathogenesis. It is now widely appreciated that the oligosaccharide portions of glycopeptides and glycolipids mediate recognition between cells and cells, between cells and ligands, between cells and the extracellular matrix, and between cells and pathogens.
These recognition phenomena can likely be inhibited by oligosaccharides having the same sugar sequence and stereochemistry found on the active portion of a glycoprotein or glycolipid involved in cell recognition. The oligosaccharides are believed to compete with the glycopeptides and glycolipids for binding sites on receptor proteins. For example, the disaccharide galactosyl .beta. 1-4 N-acetylglucosamine is believed to be one component of the glycopeptides which interact with receptors in the plasma membrane of liver cell. Thus, to the extent that they compete with potentially harmful moieties for cellular binding sites, oligosaccharides and other saccharide compositions have the potential to open new horizons in pharmacology, diagnosis, and therapeutics.
In mammalian systems, eight monosaccharides activated in the form of nucleoside mono- and diphosphate sugars provide the building blocks for most oligosaccharides: UDP-Glc, UDP-GlcUA, UDP-GlcNAc, UDP-Gal, UDP-GalNAc, GDP-Man, GDP-Fuc and CMP-NeuAc. These are the intermediates of the Leloir pathway. A much larger number of sugars (e.g., xylose, arabinose) and oligosaccharides are present in microorganisms and plants.
Two groups of enzymes are associated with the in vivo synthesis of oligosaccharides. The enzymes of the Leloir pathway are the largest group. These enzymes transfer sugars activated as sugar nucleoside phosphates to a growing oligosaccharide chain. Non-Leloir pathway enzymes transfer carbohydrate units activated as sugar phosphates, but not as sugar nucleoside phosphates.
Two strategies have been proposed for the enzyme-catalyzed in vitro synthesis of oligosaccharides. See Toone et al, Tetrahedron Reports (1990) (45)17:5365-5422. The first strategy proposes to use glycosyltransferases. The second proposes to use glycosidases or glycosyl hydrolases.
Glycosyltransferases catalyze the addition of activated sugars, in a stepwise fashion, to a protein or lipid or to the non-reducing end of a growing oligosaccharide. A very large number of glycosyltransferases appear to be necessary to synthesize carbohydrates. Each NDP-sugar residue requires a distinct class of glycosyltransferase and each of the more than one hundred glycosyltransferases identified to date appears to catalyze the formation of a unique glycosidic linkage. To date, the exact details of the specificity of the glycosyltransferases are not known. It is not clear, for example, what sequence of carbohydrates is recognized by most of these enzymes.
Much hope has been put on future developments in genetic engineering (i.e., cloning) of enzymes, particularly since several glycosyltransferases have already been cloned, including galacto-, fucosyl-, and sialyltransferases. It is hoped that future advances in cloning techniques will speed the cloning of other glycosyltransferases and enhance their stability.
Accordingly, in light of their potential uses and the difficulty or impossibility to obtain them in sufficient quantities, there exists a long-felt need for specific synthetic methods for the production of specific oligosaccharides, polysaccharides and glycopeptides and similar species in an efficient, cost effective, stereospecific, and generally applicable manner.